This essay investigates the institution of necessity in international environmental law. The point of departure is the analysis of Article 25 of the International Law Commission (ILC) Articles on State Responsibility regarding its applicability to the particular field of international environmental law. The special feature of the plea of necessity in international environmental law is that it has developed through the case-law. Without doubt, the plea of necessity can be invoked in international environmental law. However, (as in other areas of international law), it remains an exception. This was confirmed by the International Court of Justice which made clear that the invocation of necessity is very problematic. There are some unresolved issues concerning the plea of necessity in international environmental law, such as the question of the precautionary principle. The inclusion of the precautionary principle in the concept of necessity appears to give rise to a certain degree of difficulty, mainly deriving from the seemingly irreconcilable requirements of the ‘imminence of peril’ and scientific uncertainty represented by the precautionary principle. In the view of the present author, the complex and exceptional legal character of necessity in general and as applied in international environmental law in particular, will deter States from invoking it; therefore, the available practice will remain very scarce and to a certain degree inconclusive, as has already been evidenced by the available case law, analyzed in this essay.